Organic Chemistry branch of chemistry in which carbon compounds and their reactions are studied. A wide variety of classes of substances—such as drugs, vitamins, plastics, natural and synthetic fibers, as well as carbohydrates, proteins, and fats—consist of organic molecules. Organic chemists determine the structures of organic molecules, study their various reactions, and develop procedures for the synthesis of organic compounds. Organic chemistry has had a profound effect on life in the 20th century: It has improved natural materials and it has synthesized natural and artificial materials that have, in turn, improved health, increased comfort, and added to the convenience of nearly every product manufactured today.
The advent of organic chemistry is often associated with the discovery in 1828 by the German chemist Friedrich Wöhler that the inorganic, or mineral, substance called ammonium cyanate could be converted in the laboratory to urea, an organic substance found in the urine of many animals. Before this discovery, chemists thought that intervention by a so-called life force was necessary for the synthesis of organic substances. Wöhler's experiment broke down the barrier between inorganic and organic substances. Modern chemists consider organic compounds to be those containing carbon and one or more other elements, most often hydrogen, oxygen, nitrogen, sulfur, or the halogens, but sometimes others as well.


Organic Formulas and Bonds
The molecular formula of a compound indicates the number of each kind of atom in a molecule of that substance. Fructose, or grape sugar (C6H12O6), consists of molecules containing 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Because at least 15 other compounds, however, have this same molecular formula, to distinguish one molecule from another, a structural formula is used to show the spatial arrangement of the atoms:


Even an analysis that gives the percentage of carbon, hydrogen, and oxygen cannot distinguish C6H12O6 from ribose, C5H10O5, another sugar in which the ratios of elements are the same, namely 1:2:1.
The forces that hold atoms together in a molecule are chemical bonds, of which there are three types: ionic, covalent, and metallic (see Chemical Reaction; Metals). Ionic bonds are held together by the attraction of opposite electric charges. Covalent bonds are shared pairs of electrons. Wöhler's experiment, for example (see Fig. 1), resulted in a change from ionic bonds in ammonium cyanate to covalent bonds in urea. In ammonium cyanate, the attraction between the group of five atoms in NH4+ bearing a positive charge and the group of three atoms in CNO- bearing a negative charge constitutes an ionic bond. Within the NH4+ group, the four lines—N to H—represent covalent bonds, or electron pairs. Likewise, within the CNO- group and in the molecule of urea the lines represent covalent bonds. The application of heat to ammonium cyanate molecules results in a rearrangement of the bonds. The ability of carbon to form covalent bonds is not unique, as is evident from this example. The bonds between nitrogen and hydrogen are also covalent.
The ability of carbon to form covalent bonds with other carbon atoms in long chains and rings, however, does distinguish carbon from all other elements. Other elements are not known to form chains of greater than eight like atoms. This property of carbon, and the fact that carbon nearly always forms four bonds to other atoms, accounts for the large number of known compounds. At least 80 percent of the 5 million chemical compounds registered as of the early 1980s contain carbon.
Classification and Nomenclature
The consequences of the unique properties of carbon are manifest in the simplest class of organic compounds—the aliphatic, or straight-chain, hydrocarbons.
Alkanes

The parent compound of this family, the alkanes, is methane, CH4. The next members of the family are ethane (C2H6), propane (C3H8), and butane (C4H10), so the general formula for any member of this family is CnH2n+2. For compounds containing more than four carbon atoms, Greek prefixes are used with the ending -ane to name the compounds pentane, hexane, heptane, octane, and so on.
The names butane, pentane, and so on, however, do not by themselves specify molecular structure. Two different structural formulas, for example, can be drawn for the molecular formula C4H10. Compounds with the same molecular formula but different structural formulas are called isomers. In the case of butane, the common isomer names are normal butane (written n-butane) and isobutane. Urea and